The present invention is related to color image printing, and more particularly to trapping methods and arrangements that reduce the potential for unwanted image defects due to mechanical registration errors in multiple-stage color image printing systems.
Multiple-stage color image printers, such as, for example, conventional color laser printers, operate by transferring several image planes to paper in stages. Each image plane is printed with a different color ink (toner). For example, a four-stage color printer typically applies cyan (C), magenta (M), yellow (Y), and black (K) image planes during separate stages. These image planes can be created using a single drum for 4-pass printers, or one four drums for in-line printers. The image planes can be transferred to an intermediate drum or belt, and then to the paper, or directly to the paper with four transfers. Since the four image planes are created in four separate mechanical operations, it is difficult to perfectly align the four image planes on the paper so that no gaps and/or halos appear between and or near color edges. Gaps of only two 600 dpi pixels, for example, can form visible white lines in the printed image. Mechanical alignment of the image planes to an accuracy of better than {fraction (1/1000)} inch is required to eliminate such registration errors. Unfortunately, providing this level of mechanical accuracy tends to be very difficult and expensive.
Thus, for most printers, gap/halo formation problems due to registration errors are not directly addressed, but are instead left for the user/application to solve.
Trapping is the process of intentionally overlapping color edges so that small registration errors do not create gaps/halos. Trapping is commonly used in offset printing. However, this type of trapping is typically done by skilled graphics artists using special professional printing applications in the course of building up each printed image. In the more common desktop publishing setting, such valuable resources are scarce.
Thus, there is a need for automated methods and arrangements that can be used to provide advanced trapping techniques without requiring the user to provide specialized printing skills and/or other advanced trapping solutions. Preferably, the methods and arrangements will automatically operate in the background printing process in an efficient manner and without significant input from the user. The methods and arrangements should also be configured to provide trapping for color images generated by or otherwise provided through a variety of different desktop publishing, drafting, drawing, and other like applications/tools.
The present invention provides automated methods and arrangements that employ advanced trapping techniques to modify print image data prior to printing. The trapping techniques of the various methods and arrangements are configured to operate without significant user inputs and can be readily applied to all types of printable images regardless of origin. The trapping techniques employ a trapping process that is designed to detect edges between different color regions that are susceptible to gap/halo formation due to registration errors. Once detected, the trapping process selectively modifies the print image data associated with the color regions near the edge to provide an overlap of color planes that significantly reduces the potential for gap/halo formation in the subsequently printed image.
Thus, in accordance with certain aspects of the present invention, the above stated needs and others are met by a method that includes buffering at least one array of print image data that is associated with a color image. The color image includes a first color region and an adjacent second color region with at least one edge formed there between. The method further includes categorizing at least a portion of the array by color, detecting the edge based on the categorized colors, and selectively modifying the print image data in the first color region located near the edge to provide trapping, based on print image data in the second color region that is also located near the edge. The method can also include printing the modified print image data in the first color region to a print media using at least one ink. This method can be repeated for each applicable print image data in each of a plurality of color plane printing stages.
In this manner, for example, when the second color region is significantly lighter in tone than the first color region, then the color in the second color region can be pulled over the edge and trapped under the color in the first color region. If the first and second color regions are close in tone to one another, then both colors can be pulled over the edge and trapped under the other. If the second color region is significantly darker in tone than the first color region, then first color region can be pulled over the edge and trapped under the color in the second color region.
An apparatus is also provided, in accordance with certain aspects of the present invention. The apparatus includes a converting unit that is configured to receive color image data and output corresponding print image data. The print image data is provided to a trapping unit that is configured to selectively modify and output portions of the print image data associated with a portion of a first color region located near an edge with a second color region, based on print image data associated with a portion of the second color region located near the edge. The apparatus further includes a printing unit that is configured to selectively apply ink to a print media based on the modified print image data. The apparatus can be embodied in hardware, software, firmware, and/or any combination thereof.
In accordance with still further aspects of the present invention, a system is provided that includes at least one computer, or like device, and a color printer. The computer is configured to output color image data that defines a first color region and an adjacent second color region. The color printer, which is connected to the computer, is configured to receive the color image data and output a corresponding printed image that depicts the first color region and the second color region and at least one visible edge formed there between. Trapping of colors near the edge is provided within the color printer by a trapping unit similar to that in the apparatus described above. As such, trapping techniques can be automatically considered and where appropriate applied to the print image data, regardless of the type of originating application in the computer or like device. Furthermore, a plurality of computers can benefit by a single embodiment of the trapping process within a shared color printer.